The pectoral region is located on the anterior chest wall. It is an important area of the body that contains four muscles that interact with the upper limb: the pectoralis major, pectoralis minor, serratus anterior, and subclavius. This article will explore the anatomy of the muscles of the pectoral region, the attachments, actions, and innervations associated with each.
The pectoralis major is the most superficial muscle of the pectoral region. It is a large, fan-shaped muscle composed of two distinct parts: a clavicular head and a sternal head. The clavicular head originates from the anterior surface of the medial clavicle, while the sternocostal head originates from the anterior surface of the sternum, the superior six costal cartilages, and the aponeurosis of the external oblique muscle. These two parts of the muscle combine to insert on the intertubercular sulcus of the humerus. The primary actions of the pectoralis major are adduction and medial rotation of the upper limb and a drawing of the scapula in an anteroinferior direction. Furthermore, the clavicular head acts individually to flex the upper limb. The pectoralis major is innervated by the lateral and medial pectoral nerves.
The pectoralis minor sits underneath the pectoralis major and forms part of the anterior wall of the axilla region. It originates from the third to the fifth ribs and inserts into the coracoid process of the scapula. Its primary action is to stabilize the scapula by drawing it anteroinferiorly against the thoracic wall. It is innervated by the medial pectoral nerve.
The serratus anterior is featured more laterally in the chest wall and forms the medial border of the axilla region. It is composed of several strips of muscle that originate from the lateral aspects of the ribs one to eight and attach to the costal (rib-facing) surface of the medial border of the scapula. Its primary action is to rotate the scapula, allowing the arm to be raised above 90 degrees, and to hold the scapula against the ribcage. It is innervated by the long thoracic nerve.
The subclavius is a small muscle located directly underneath the clavicle, running horizontally. Its primary purpose is to provide some minor protection to the underlying neurovascular structures in the event of clavicle fracture or other trauma. It originates from the junction of the first rib and its costal cartilage and inserts onto the inferior surface of the middle third of the clavicle. Its primary action is to anchor and depress the clavicle. It is innervated by the nerve to subclavius.
One of the actions of the serratus anterior is to 'hold' the scapula against the ribcage. If the long thoracic nerve is damaged or paralyzed, this holding action is no longer present. In such cases, the scapula protrudes out of the back and has a 'winged' appearance, a clinical sign referred to as 'winging of the scapula.' Long thoracic nerve palsy is most commonly thought to occur due to traction injuries, such as those associated with violent stretching of the upper limb.
